1. Field of the Invention
The present invention relates to power line communications, and more particularly, to a data coupler being insulated in a manner that minimizes voltage breakdowns.
2. Description of the Related Art
An inductive coupler for power line communications couples a data signal between the power line and a communication device such as a modem. The inductive coupler may suffer from insulation breakdown or partial discharge at unsuitably low line voltages. Breakdown or partial discharge will generally occur at a location within the coupler where an electric field is concentrated in an insulating material or where an excessively high field is created through the air.
FIG. 1 shows a cross-section of a prior art inductive coupler. A power line 800, e.g., a phase line, serves as a primary winding for the inductive coupler, and thus passes through an aperture of a magnetic circuit with a core configured with an upper core portion that includes a core section 805 and a lower core portion that includes a core section 810, and air gaps 830 and 835. A secondary winding 820 also passes through the aperture, surrounded by an insulating material 825. Power line 800 touches core section 805 at a contact point 855, while secondary winding 820 is grounded. Core sections 805 and 810 are made of a magnetic core material. An electric field inside of core sections 805 and 810 depends on conductivity and permittivity of the core material.
For the case of power line 800 being bare, the full phase voltage is applied to the coupler, specifically between contact point 855 and secondary winding 820.
Referring to FIG. 2, for the case of power line 800 being covered with insulation, there is shown power line 800 having insulation 860 that contacts core section 805 at a contact point 865. A capacitive voltage divider is formed between (a) a capacitor formed between power line 800, insulation 860, and core section 805, and (b) a capacitance between contact point 865 and secondary winding 820. The voltage stress between contact point 865 and ground is then less than the full phase voltage.
A plane where secondary winding 820 exits core section 810, core section 810 presents a sharp corner. In general, there may be two locations susceptible to ionization and voltage breakdown, (1) an air path 840 between power line 800 and insulating material 825, and (2) a region between of the corners of core section 810 and the exiting of secondary winding 820 from core section 810.
Air path 840 is susceptible to ionization and voltage breakdown, as follows. Insulating material 825 is likely to be constructed from a plastic or other material with a permittivity of 2.5-3.5. A capacitive voltage division of a voltage difference between power line 800 and secondary winding 820 will place most of the voltage difference in air path 840, and relatively little of the voltage difference across an insulation path 850. The insulating capability of air is inferior to that of plastic or other insulating material, so as voltage on power line 800 increases, a breakdown is most likely across path 840.
FIG. 3 shows a horizontal cross section drawn through a secondary winding 820 such as that shown in FIG. 1. The lower core portion is shown as being configured with a plurality of core sections, namely core sections 810, 811, 812 and 813. Secondary winding 820 passes through core sections 810, 811, 812 and 813. Regions 1000, 1005, 1010 and 1015 represent regions of electric field concentration, and might cause initial insulation breakdown at a voltage on power line 800 that is much lower than desired.